1. Field of the Invention
The invention relates to the field of discharge valve apparatus for a tank, and particularly a valve apparatus for use with a tanker truck or the like of the type commonly used for transporting bulk gasoline or other liquid.
2. Description of the Prior Art
Existing valves for tanker trucks, as exemplified by the valve disclosed in my U.S. Pat. No. 3,420,495, patented Jan. 7, 1969, and entitled "Tank Valve Apparatus," are characterized by an actuating cylinder operative upon a linkage system to open the tank valve against the bias of a tank valve spring. Such a cylinder includes a single acting piston engaged upon the walls of the cylinder and movable to its extended position to open the valve by a force which is the product of the cylinder pressure and the area of the piston.
Such a cylinder presents maintenance problems because eventual scoring of the cylinder walls is caused by continued reciprocation of the piston. In addition, the friction of the piston upon the cylinder walls typically requires use of a return spring to initiate return movement of the piston upon release of pressure to the cylinder to close the tank valve. Relatively rapid return movement of the piston is desirable to allow the tank valve closing spring to forceably close the valve and thereby provide optimum seating.
In addition to the foregoing problems presented by prior art air cylinders, many of the prior art tank valve systems employ a relatively complex, high friction linkage means for opening the tank valve. The number of parts and the manner of their interaction is such that higher than desirable air pressures are necessary to operate such linkage means. This, in turn, requires that tanker trucks carry larger air compressors or operate the compressors more continuously.